Affinity distance in maize: Effects of mitogens on chromosome orientation

The effects of anti-mitotic agents on the 'affinity-distances' of metaphase chromosomes of maize have been under study in this lab for several years (J. D. Horn, MNL 45:209, 1971; J. D. Horn and D. B. Walden, Genetics 88:181, 1978). The present investigation is an attempt to refine this analysis by employing new methods of data collection and analysis.

The earlier methodology involved using pair-wise comparisons of the mean distances between all possible combinations of homologous and non-homologous chromosomes in no fewer than twenty-five metaphase spreads for each experimental condition. These values, in addition to describing the mean distance between each pair of chromosomes, can be compared to the value derived by J. Hammersley (Ann. Math. Stat. 21:447, 1950) and R. D. Lord (Ann. Math. Stat. 25:794, 1954) for the mean distance between points randomly distributed in a sphere pressed into a circle. Random and non-random associations can thus be distinguished.

In our newer methodology, we record the positions of the chromosomes as the (x,y) coordinates of their centromeres and telomeres from "8x10" photographs of standard spreads. The recording is done on a computerized digitizer. As a point of reference, the centroid of the figure is calculated as are all distances between the centroid and the telomeres and centromeres. This allows the orientation of the chromosomes to be determined and offers another approach to the overall question of how the nucleus may be organized.

Seeds of the inbred Oh43 were germinated in the dark for 72 to 96 hours then divided into two groups for treatment with mitogens. Roots from group one were excised prior to treatment. In group two, the roots were not excised until treatment and fixation were complete. The treatments included cold (5 C for 24 hours), 8-hydroxyquinoline (0.003% for 3 hours), monobromonaphthalene (2.0% for 3 hours), and colchicine (3x10-4 M for 3 hours). In both groups the controls consisted of untreated roots placed directly into fixative. No difference has been shown to occur between groups one and two.

The orientation of chromosomes 6 and 10 have been examined. Nearly 2/3 of the 6S arms are oriented toward the centroid of the spread. This orientation is not altered by a mitogen, confirming our earlier suggestion (Horn and Walden, 1978) that the position of chromosome 6, unlike all other chromosomes of the complement, was insensitive to an alteration in the microtubules. Chromosome 10S orientation towards the centroid, however, was markedly reduced in mitogen treated material as compared to the cold arrested treatment.

Additional pattern analysis of these data and data from cells with and without B chromosomes is underway. It is clear that both distance and position parameters of a pair of homologues are specific for that homologue and are in some cases altered by the action of one or more mitogens.

D. J. deKergommeaux and D. B. Walden

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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